(1) Field of Invention
This invention relates to dry surface CO2 detectors. More specifically it relates to rapid-response reversible dry surface CO2 detectors prepared from a solution comprising a poly(oxyethylene) based compound, a pH sensitive color indicator dye, an organic solvent, a cationic phase transfer agent, and an anionic base.
(2) Description of Prior Art
Dry surface CO2 detectors are known in the prior art. They have numerous applications including uses in industrial monitoring, environmental monitoring and in medicine.1 Accurate detection of particular threshold concentrations of CO2 in gaseous samples can be of critical importance in the medical field, particularly when attempting to confirm the proper placement of an endotracheal tube in the airway of a patient. An appropriate properly calibrated CO2 detector which can be inserted into the air path allows one to distinguish between an endotracheal tube placed properly into the airway, through which respiratory concentrations of expired CO2 will be detected, and a tube improperly placed into the esophagus through which no expired CO2 will normally be detected. The difference between a properly and improperly placed endotracheal tube can very quickly become a life or death matter and so the ability to quickly, easily, safely and accurately distinguish between the relevant concentrations of CO2 is highly desirable.
The dry surface CO2 detector provides a great advantage over previous technologies such as the Einstein CO2 detector because the Einstein CO2 detector utilizes a liquid CO2 detecting solution, and is therefore far less versatile and can even present certain hazards when improperly used. When appropriate safe chemistry is used, the dry surface CO2 detector can be readily be placed into the air path in any of a number of configurations without any concern that the patient will aspirate any harmful CO2 detecting materials. The dry surface detector may be placed for example, either inside the endotracheal tube, inside an extension of the endotracheal tube, into a resuscitator bag assembly, or into a specially designed holder which is designed to connect into the air path of the patient such as the single patient use carbon dioxide detector as described in U.S. Pat. No. 6,502,573, herein incorporated by reference.
Further, a rapid-response reversible dry surface CO2 detector which will repeatedly not only rapidly indicate the presence of respiratory concentrations of CO2 (4.5%-5%) but which will also rapidly indicate the change back to ambient CO2 concentration (˜0.03%) is even more desirable than a single response detector for endotracheal intubation purposes. Such a rapid-response reversible detector can not only confirm initial proper placement of the endotracheal tube but can also confirm continued proper placement of the tube. With each exhalation, respiratory concentrations of CO2 will be exhaled through a properly placed tube while with each inhalation, ambient concentrations of CO2 will be inhaled through the properly placed tube. With a detector that will repeatedly indicate the change from ambient to respiratory concentrations of CO2 and back again, it becomes possible to verify breath by breath that the patient is continuing to breathe properly through the tube.
Another important point is that using the method of checking for increased concentrations of expired CO2 through an intubation tube as a means of verifying correct tube placement can result in a rare false positive initial test for proper intubation. This may occur when there is increased CO2 concentration present in the esophagus due to, for example, recent consumption of carbonated beverages, which may lead to CO2 being expelled through an intubation tube that has been improperly inserted into the esophagus, thus giving a false positive initial indication regarding the success of the intubation. In such a situation a rapid-response reversible detector that continuously responds to both the upward and downward changes in CO2 concentration in the air path would quickly subsequently reveal when tube placement is improper. However, in this unusual situation, a single-response non-reversible detector would only indicate the initial false positive (high concentration of CO2 present) for correct intubation but would not give any further information, thus likely quickly leading to a very dangerous situation.
To effectively serve the function of assisting in verifying proper initial placement of an endotracheal tube, the detector must respond to the relevant changes in CO2 concentration within an appropriate respiratory timeframe, meaning no more than 20 seconds at the slowest (See U.S. Pat. No. 5,166,075). So non-reversible detectors designed to give a one-time response indicating the presence or absence of CO2 could function successfully for the intended purpose even when they respond relatively slowly.
To serve the function of verifying continued proper placement of an endotracheal tube, a CO2 detector must respond to the relevant changes in CO2 concentration within a shorter timeframe. Assuming a respiratory rate of 15 breaths/minute for a healthy adult (see U.S. Pat. No. 6,436,347), such an application requires that clear indications of the relevant changes in CO2 concentration be given by the detector within 2 seconds of exposure to a changed concentration of CO2 in order to observe breath-by-breath changes clearly. However sick or stressed adults may have a faster respiration rate. Children have an average respiration rate of 20 to 40 breaths/minute and newborns have an average respiration rate of 30 to 60 breaths/minute. And of course, a faster respiratory rate would shorten the acceptable detector response time (0.75 to 1.5 seconds for children, 0.5 to 1.0 seconds for newborns). Therefore a reversible CO2 detector with a one-way response time faster than 0.5 seconds is preferable so as to provide the necessary function in virtually any medical situation that might be encountered.
U.S. Pat. No. 5,005,572 describes the production of dry surface CO2 detectors employing a color indicator dye and a phase transport enhancer. Several examples are given of functional CO2 detectors which will change color so as to indicate the presence of a given concentration of CO2 in a gaseous sample. These detectors incorporate a phase transport enhancer into their composition. A broad range of possible phase transport enhancers is claimed, anything of the form:
whereX=N or P;R1, R2, R3 and R4 are selected from the group consisting of C1-C12 alkyl, C1-C4 substituted alkyl wherein the substituent is a C1-C4 alkyl or phenyl group, naphthyl, benzyl, and pyridine;R5 is selected from the group consisting of C1-C12 alkyl or benzyl; andY− is an anion selected from the group consisting of hydroxide, fluoride, chloride, bromide, iodide, carbonate and tetrafluoroborate.
In U.S. Pat. No. 5,005,572, only a few examples are given of reversible CO2 detectors which will not only change color in response to the presence of respiratory concentrations of CO2 (4.5-5%) but which will also substantially revert back to their initial color when subsequently exposed to a lower ambient concentration of CO2 (˜0.03%). The particular examples given all utilize a combination of TBAH (tetrabutylammonium hydroxide) as a phase transport ‘enhancer’ in combination with one of several pH sensitive indicating dyes in their formulations. However, no specific time frames for response of the ‘rapid’ detectors are given which would allow evaluation of the suitability of these particular detector compositions for the purpose of monitoring ongoing respiration and/or monitoring continued correct placement of an endotracheal tube.
U.S. Pat. Nos. 4,728,499; 4,994,117; 5,166,075; and 5,179,002 disclose examples of CO2 detectors produced by drying an indicating solution onto a carrier. The indicating solution is aqueous and/or non-volatile and is comprised of a basic solution, a pH sensitive indicator dye, and a high boiling, water miscible hygroscopic liquid but does not make use of phase transport enhancers. Workable CO2 detectors are disclosed but among these patents, only one example, given both in U.S. Pat. No. 5,166,075 and in U.S. Pat. No. 5,179,002, provides for a reversible detector which will indicate the change from ambient to respiratory concentrations of CO2 as well as the change back to ambient CO2 concentrations. This example utilizes a combination of water, sodium carbonate, glycerol and m-cresol purple. Upon exposure to 5% CO2, a detector made according to this example will give a color-change response in 5 seconds and will turn back to its original color quickly upon re-exposure to ambient CO2 concentrations. These response times may be fast enough to be useful in verifying initial correct placement of an endotracheal tube but will not be fast enough to be reliable for determining continued correct placement of an endotracheal tube which may require response and reverse-response times to each be on the order of 2 seconds or less.
U.S. Pat. No. 6,436,347 describes the use of quaternary ammonium and quaternary phosphonium phase transport enhancers to produce fast response calorimetric indicators which are substantially insensitive to humidity. Specific examples are given using the specific phase transport enhancers tetraoctylammonium hydroxide, trimethylhexadecylammonium hydroxide, and tetradecyltrihexylammonium hydroxide together with Thymol blue to produce functional CO2 detectors. However, beyond a single mention of the word ‘reversible’ in column 1 of the ‘Background of the Invention’ section, no evidence is given of a reverse-response by the detector when the higher CO2 concentration gaseous sample (5% CO2) is subsequently replaced with ambient air (0.03% CO2). Without an equally swift reverse-response when CO2 concentrations return to ambient levels of 0.03%, these fast response detectors would not be suitable to accomplish the continued monitoring of correct placement of an endotracheal tube.
Though the examples given in U.S. Pat. No. 6,436,347 utilize only tetraoctylammonium, trimethylhexadecylammonium, or tetradecyltrihexylammonium cations, coupled with a hydroxide anion, the claims are much broader including any phase transport enhancers of the form:
where X is a nitrogen or phosphorus atom; andwhere R1, R2, R3, and R4 are alkyl groups and at least one of the alkyl groups has at least 13 carbons, and at least one of the other alkyls have 6 to 8 carbons, and the remaining alkyls (if any) have 1 to 12 carbons;Y− is an anion selected from the group consisting of hydroxide, fluoride, chloride, bromide, iodide, carbonate and tetrafluoroborate.